Inside a vacuum enclosure, an X-ray tube comprises, a cathode constituted by a heated filament which emits electrons, and a focusing device backed onto the filament which focuses the emitted electrons onto an anode raised to a positive potential relative to the cathode. The point of impact of the electron beam on the anode constitutes the source of X-radiation in the form of a beam.
In order to deflect the X-ray beam angularly, proposals are generally made to displace the point of impact of the electron beam over the anode by using deflecting means. Such deflecting means are generally constituted by magnetic or electrostatic lenses which are disposed on the path of the beam or in the vicinity of said path between the cathode and the anode. Because of the high kinetic energy of the electrons in the beam, and their high speed given the high potential difference between the cathode and the anode (more than 100 kilovolts), using such lenses consumes a non-negligible amount of energy.
French patent number 2 538 948 proposes a scanning X-ray tube in which the focusing device includes at least two metal parts which are electrically insulated from each other and from the filament in order to enable them to be biased independently relative to the filament and thus deflect the electron beam.
FIG. 1 is a schematic view of an X-ray tube of the type described in the above-mentioned patent application. Inside a vacuum enclosure represented by a dashed line rectangle 11, it comprises a filament 12, a focusing device 13 backing onto the filament 12, and an anode 14. The filament 12 and the focusing device 13 constitute a cathode C1. The focusing device 13 is constituted by a first metal part 15 and a second metal part 16 which are electrically insulated from each other by an insulating partition 17 fixed to an insulating base 18. The metal parts 15 and 16 are disposed symmetrically on either side of the filament 12 about a plane of symmetry extending perpendicularly to the plane of FIG. 1. This plane of symmetry contains the axis of the filament 12 which extends perpendicularly to the plane of FIG. 1, and is perpendicular to the base 18. The axis 19 of the electron beam is defined by the intersection of said plane of symmetry with the plane of FIG. 1.
When equal voltages are applied to the metal parts 15 and 16, the cathode C1 emits an electron beam F along the axis 19, with the beam being focused by the geometry of the cathode 1.
In order to deflect the electron beam, i.e. in order to impart a mean direction thereto different from the emission axis 19, it is sufficient to set up asymmetry in the electric field created around the filament 12 by applying voltages of different values to the metal parts 15 and 16, one of which values may be zero, but neither of which may be positive. A beam F' having an axis 19' can thus be obtained for a positive potential difference between the part 15 and the part 16; similarly, a beam F" having an axis 19" is obtained for a negative potential difference between the part 15 and the part 16.
The X-ray tube described above with reference to FIG. 1 provides satisfactory deflection performance without requiring high bias voltages to be applied to the metal parts 15 and 16. However, such an X-ray tube provides only one X-ray beam having determined energy characteristics. However, in some applications, two X-ray beams are required having different energy characteristics and capable of taking up various angular positions while retaining a determined width.
To this end, it is known to make cathodes having two filaments supplied with currents of different values. These filaments may be in axial alignment so as to use the same metal focusing parts, but this causes the point of impact of the electron beam on the target to be displaced along the filament axis and also changes the width of said point of impact.
In other embodiments, instead of being aligned in the same groove, the two filaments are disposed in respective parallel grooves separated by a metal partition. In this way, the point of impact of each electron beam is at a position in the longitudinal direction of the filaments which is substantially the same for both filaments. However, the position of the point of impact along a direction transverse to the said filament axis differs from one filament to the other, as does the concentration of the beam. In addition, it is difficult to obtain deflection of the beams using metal parts.
An object of the present invention is to provide a cathode for an X-ray tube including at least two filaments disposed in separate mutually parallel grooves and which enables each electron beam to be deflected while simultaneously controlling concentration thereof.